This invention relates to method and apparatus for monitoring and detecting strains and deformations in solids, and more particularly to method and apparatus adapted to detect strains within subterranean formations.
In accordance with one aspect, the invention relates to a strainmeter instrument and strain monitoring system capable of operating at great subterranean depths and measuring rock formation movement without being adversely affected by high temperatures in the rock formation.
The use of borehole strainmeters and systems to measure movement of subterranean rock formations is known. Many different strainmeters and systems have been devised for commercial use, but most have defects and limitations in operation, especially under extreme borehole depth conditions.
One known borehole strainmeter comprises a resilient metal tube prestressed into a borehole with an expansive grout so that the tube follows the deformation of the rock exactly. The deformation of this is amplified by hydraulic means and drives a small, thin-walled bellows, which in turn drives an electronic transducer. All power and signals reach the surface through an electric cable. There are a number of these devices in use worldwide.
However, because culturally induced noise decreases with depth, and because in many critical areas, e.g., Tokyo alluvial cover can be kilometers thick, and because earthquake faults lock at depth, there is a continuing and increasing need for more sensitive detectors capable of operating at multi-kilometer depths where temperatures are too high for electronics to survive for long periods of time.